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Porosity, Why?

I ran out of gas for my mig so I thought it would be a good time to learn some on tig. The photo shows my problem, porosity. The metal was clean of all mill scale, the cuts were with a band saw and there was no oil on it. I thought maybe I had the flow rate to low on the argon, it was at 20 cfm so I upped it to 30 cfm, no help. The tungeston was 2% thoriated, i made sure not to dip it. I did notice that the tip was yellow after I would weld.

Not sure of the cup size, I'd guess about 5/8" or 1/2". There was no air movement at all, it seems that all of them start out fine but about half way through the porosity starts. The welds are in a non critical area, they're ugly know buy I have to learn sometime.

In my opinion the most likely culprit would be gas coverage. Outside corner welding like you have there is often the joint that will show up gas coverage problems. The problem lies in that the gas from the torch tends to slide straight past the weld joint as it is the path of least resistance, this can then draw air along with it as in fact you now have a low pressure area.
The common mistake is to increase gas flow but this simply increases the speed of gas slipping past the weld joint which in turn is an even greater low pressure area drawing in more air.
Gas lenses slow down the gas flow giving a much more even gas coverage and I love them to bits. If you don't have any gas lenses try decreasing your gas flow out the torch and shorten the amount of tungsten stickout.
This technique has worked well for me in the past but now I use gas lenses exclusively (except for Magnesium welding ) and the problem doesn't crop up like it used to.
Regards Andrew from Oz.

The rods are 706s and the torch has a regular cup on it. On the tungeston stick out, it's 3/16" to 1/4". I'll try the lower flow rate and see what happens. I had the same problem with the weld on a 1/2" shaft with a 3/8" thick flat washer welded to it for a slide hammer I'm building. I'm going to figure this thing out if it's the last thing I do, in the mean time, Monday I'll get my mig bottle filled.